1. Field of the Invention
The present invention relates to the field of broadband wireless radio frequency (RF) communications, and in particular to a reconfigurable impedance match circuit, which may be used in broadband wireless devices and a method for selecting component values for analog circuits.
2. Discussion of the Background
In the past decade the need for broadband communications has increased rapidly. With this increased need the inadequacies of current systems has become apparent. In order to increase performance much research has been done on different modulation schemes and codecs, different antennas, and transmission circuits. An often overlooked but potentially highly limiting factor in the bandwidth performance of a system is the impedance match between important elements in the system.
Conventional impedance matching solutions are often accomplished in a static sense. For example, the impedances of transmission circuits and antennas may be calculated at design time and a static matching case may be built into the design. However, this approach may not adequately account for significant circuit element impedance changes that may occur during the life of the system, which may invalidate the static matching case.
For example, a cellular phone antenna may have clearly defined impedance parameters in its nominal state to which the static matching structure may be designed. If the user were to place a hand over the antenna during operation, the reactive impedance of the antenna would greatly change. In order for the transmission system to function correctly it must radiate a certain amount of energy. Since the antenna impedance is now changed, much of the energy is reflected back to the transmission circuit from the antenna, resulting in a lower radiated energy from the antenna. Since the cellular phone needs to radiate a certain amount of energy and less is now being radiated due the impedance mismatch, the phone reacts by increasing the output from the transmitting circuit, resulting in a an efficiency decrease, which may not be prevented when using a static matching network.
Reconfigurable matching networks can be changed if a certain matching case is no longer valid. In recent years there has been quite a bit of interest in circuits utilizing MEMS (Micro-Electromechanical Systems) technology. MEMS devices often use switches and capacitors in a matching network to change the performance of a periodic structure.
J. Papapolymerou, et al., “Reconfigurable Double-Stub Tuners Using MEMS Switches for Intelligent RF Front-Ends,” IEEE Trans. Microwave Theory and Techniques, vol. 51, no. 1, January 2003, which is incorporated herein by reference in its entirety, describes a simple two stub impedance matching network using MEMS that may have interesting properties. This double stub tuner can be configured to match a fairly wide range of loads. Reconfiguring the structure is accomplished by capacitive loading of the two stubs in the matching network. The amount of capacitive loading is determined by a bank of capacitors, selectively picked using MEMS switches. A problem with this approach lies in the aspect that a discrete set of loads can be matched. The greater the desired matching load, the larger the capacitor bank and number of required switches. Additionally the operation of the circuit may be restricted to a narrow bandwidth, estimated to be 10%-15% using λ/2 resonators, with bandwidth defined as the 3 dB attenuation point.
Later, Y. Lu, et al., “A MEMS Reconfigurable Matching Network for a Class AB Amplifier,” IEEE Microwave and Wireless Components Letters, vol. 13, no. 10, pp. 437-439, October 2003, which is incorporated herein by reference in its entirety, used the same double stub tuner approach as discussed in Papapolymerou to design a matching network for use in a power amplifier system. Since it was essentially the same circuit as proposed in Papapolymerou, the impedance matching structure proposed by Lu may also suffer from low bandwidth and discrete tuning limitations.
Hunter et al., “Electronically Tunable Microwave Bandpass Filters,” IEEE Trans. Microwave Theory and Techniques, vol. MTT-30, no. 9, pp. 1354-1360, September 1982, which is incorporated herein by reference in its entirety, describes an electronically tunable bandpass filter, which can be used as an impedance matching network. Further, Hunter describes a 5% band-pass filter having the pass band constrained to the 3 dB attenuation points. The physical realization described in Hunter includes a comb-line filter on microstrip with varactor diodes loading the ends of short circuited fingers. However, the structure of Hunter has narrow bandwidth and poor insertion loss properties (nearly 6 dB). The varactor diode in Hunter has a limited range of capacitance, which affects the reconfigurable nature of the circuit. Makimoto et al., “Varactor Tuned Bandpass Filters Using Microstrip-line Ring Resonators,” IEEE MTT-S Digest, pp. 411-414, 1986, which is incorporated herein by reference in its entirety, describes a reconfigurable band-pass filter implementation having a combination of varactor diodes and ring resonators. Makimoto mentions altered coupling between resonators but does not describe such an implementation.
Thus, conventional reconfigurable networks may rely on the user having particular advanced knowledge regarding a mismatch between load and source. In addition, it may be desirable for users to have a straightforward method for determining adjustments of the reconfigurable network needed to account for a load mismatch. Unfortunately, conventional solutions may not adequately provide methods to detect and use information regarding source and load impedance disparity.
Mingo, et al., “An RF Electronically Controlled Impedance Tuning Network Design and Its Application to an Antenna Input Impedance Automatic Matching System,” IEEE Trans. Microwave Theory and Techniques, vol. 52, no. 2, pp. 489-497, February 2004, which is incorporated herein by reference in its entirety, presents a high frequency front end system operating at 390 MHz, including an impedance matching network connected to a coupler that detects a mismatch in impedance, and an algorithm to correct the detected mismatch. However, the impedance matching network of Mingo uses a discrete tuning method much like earlier MEMS devices. Instead of MEMS switches, however, p-i-n diodes were used to activate different banks of capacitors, limiting a resulting system to function over discrete loads. Furthermore, the device described by Mingo has a narrow bandwidth, and Mingo fails to describe a detailed scheme for detecting the mismatch between source and load
Thus, Mismatches in the impedance characteristics between the source and load of many broadband applications are an often overlooked but limiting factor in the performance of a broadband system. To correct for mismatches in impedance, transformers and matching circuits are classically used. In general, impedance matching components are developed for a static sense and function only with non-varying source and load impedances. If the impedance of either the source or load changes, however, the efficiency and bandwidth characteristics can suffer as a result.